Doping Process, Photocatalytic Activity And First-Principle Calculations Of Fe Or Cr-Doped ZnO And ZnO/Ag

The wide band gap of ZnO reduced the range of sunlight response on ZnO and seriously hindered its wide spread application in the field of photocatalysis. Some study has shown that it can reduce the width of the gap, improve the separation efficiency of photo-generated electrons and holes pairs and greatly improve the utilization ratio of sunlight by doping or modifying ZnO nanomaterials. Therefore, the study on doping and modification of ZnO has become a hot topic in some areas, such as material, chemistry, environment etc. In this dissertation, Fe-doped ZnO, Fe-doped ZnO/Ag and Cr-doped ZnO nanocomposites were prepared by simple hydrothermal and solvothermal method, and characterized by several analytical techniques. The preparation technicses of Fe-doped ZnO, Fe-doped ZnO/Ag and Cr-doped ZnO nanocomposites were optimized. The photocatalytic activity and mechanism of Fe-doped ZnO/Ag and Cr-doped ZnO nanocomposites under simulative sunlight, ultraviolet and visible light irradiation were studied and discussed. In this dissertation, the main research contents as follows:(1) Fe-doped ZnO and Fe-doped ZnO/Ag nanocomposite were successfully synthesized by hydrothermal method, in which, Zn(CH3COO)2·2H2O, AgNO3and Fe(NO3)3·9H2O as raw material, C2H5OH as the reducing agent, NaOH as precipitating agent, C68O7·H2O as dispersant. The process conditions of Fe-doped ZnO/Ag with good photocatalytic activity were optimized by conditional experiment. The photocatalytic activity of the products was researched by degradation of methyl orange (MO). The products were characterized by X-ray diffraction (XRD), Field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), Selected-area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), Energy Dispersive Spectrometer (EDS) inductively coupled plasma optical emission spectrometry (ICP-OES) and ultraviolet-visible spectrophotometer (UV-vis). The results of the study indicated that the optimal molar ratio of Fe:Zn of Fe doped ZnO is1.5%. The optimal process conditions of Fe-doped ZnO/Ag were as follows:the reaction temperature was150℃the optimal molar ratios of Ag:Zn and Fe:Zn were7.5%and1.5%, respectively. The morphology of Fe-doped ZnO/Ag at the optimal process conditions is sheet, and the actual molar ratios of Ag:Zn and Fe:Zn were5.67wt.%and0.80wt.%, respectively. Ag was loaded to the surface of ZnO and iron element was coexisted in the lattice of ZnO as the Fe2+and Fe3+ionic states and substituted for some of the lattice zinc ions. When Fe-doped ZnO/Ag nanocomposites synthesized under the optimized preparation process were as photocatalyst, the degradation rate of MO solutions was99.9%under800W xenon lamp for150min and increased obviously than that of ZnO/Ag and pure ZnO.(2) Cr-doped ZnO nanomaterial was successfully synthesized by solvothermal method, in which, Zn(CH3COO)2·2H2O and Cr(NO3)3·9H2O as C2H5OH as the solvent, NaOH as precipitating agent. The process conditions of Cr-doped ZnO nanocomposites with good photocatalytic activity under visible light were optimized by conditional experiment, including the molar ratio of Cr:Zn, reaction temperature and reaction time. The photocatalytic activity of the products is decided by the photocatalytic degradation of methyl orange. The products were characterized by XRD, XPS, SEM, EDS, UV-vis and PL. The results of the study indicated that the optimal process conditions of Cr-doped ZnO nanomaterial were as follows:the reaction temperature was120℃, the reaction time was12h and the molar ratio of Cr:Zn was4%. The morphology of Cr-doped ZnO at the optimal process conditions is fluffy sphere. Chromium element is predominantly existed in the lattice of ZnO as the Cr3+ionic state and substitute for some of the lattice zinc ions. When Cr-doped ZnO nanomaterial synthesized under the optimized preparation process were as photocatalyst, the degradation rate of MO solutions was99.0%under ultravioletlight for60min and was1.3times higher than pure ZnO; the degradation rate of MO solutions was97.8%under visiblelight for4h and was3.4times higher than pure ZnO. The increasing rate of the latter is far greater than that of the former.(3) The band structure and density of states (DOS) of the doped and un-doped samples had been calculated in the frame of DFT using the CASTEP code. The results showed that the band gap of Fe-doped ZnO and Cr-doped ZnO reduce by about0.162eV and0.033eV compared to undoped ZnO, respectively. In addition, Zn3d states spread to the lower level with dispersion for Fe or Cr-doped nanocomposites. In this situation, the Fermi level shifts to the conduction band and a narrow-deep acceptor level formed in the energy gap after metal irons doping and the transition of an electron from valence band to conduction band will need less energy in Fe or Cr-doped nanomaterials which demonstrated the mechanism for the enhanced photocatalytic activities of metal irons doped nanomaterial.